This invention relates to cathode or negative discharge glow lamps and, in particular, to glow lamps producing whiter light than has been provided in the prior art.
Glow lamps having a fluorescent coating are known in the art. For example, U.S. Pat. No. 2,409,769--Leyshon describes such a glow lamp utilizing a manganese-activated zinc silicate (green) phosphor. The particular lamp described has a pronounced green color and uses less than 1% krypton in the gas at a pressure of 4.7.times.10.sup.3 pascals (35 mm Hg). Thus, the lamp does not produce as white a light as desired and tends to have a short life due to sputtering of the electrodes.
Phosphor mixtures are well known in the fluorescent lamp art to achieve the desired color of light from the lamp. However, fluorescent lamps rely on ultraviolet radiation, particularly the strong radiation at 253.7 nm, produced by a mercury arc as the source of energy for the phosphors. As is known, it is much more difficult to use mercury as an ultraviolet radiation source in a negative discharge.
While it is known to use krypton or xenon as the source of ultraviolet radiation in negative discharge devices, it is not known to use multiple phosphors in such devices and, particularly, to use one phosphor to excite another, thereby producing a whiter light.
In general, it is desirable to have a glow lamp producing white light since any narrower spectrum color is easily obtained by filtering. For example, glow lamps are predominantly used as indicators. By providing a whiter light glow lamp, one can use a single lamp type and obtain different colors simply by filtering, thereby reducing parts inventories and costs.